1. Field of the Invention
The present invention relates to an automatic focusing device.
2. Description of the Prior Art
Conventionally known automatic focusing devices are designed to generate control information representing a target for a servo system for moving a photographic lens to an in-focus position, in response to a photoelectrically converted output signal derived from an image of an object formed for example on a solid-state image sensor.
However, when such control information is directly used for servo control, the servo system may receive unexpected inappropriate control information within a very short period due to noise generated in the device, shaking of the camera or momentary movement of the object. Thus, when the photographic lens is in the vicinity of the in-focus position, the focusing device may respond to such inappropriate control signal so that the photographic lens is first brought to an out-of-focus position and then to the in-focus position. Stated differently the conventional focusing devices are subject to unstable operation when the photographic lens is in a range close to the in-focus position.
Such unstable operation can be prevented by filtering the control signal in order to eliminate the sudden inappropriate signal generated for a very short period, thus ensuring smooth servo control. In such case, however, the servo system operates with a certain delay in response, thus resulting in a reduced responsive ability of the automatic focusing device. Such delay is often not permissible, particularly outside the range close to the in-focus position.